Eag-related channels (EAG, hERG, ELK) all share a highly conserved region bearing homology with cyclic- nucleotide binding domains of a variety of other proteins including cyclic nucleotide-gated channels. However, most evidence indicates this domain has evolved to serve a different, but unknown, function in Eag-related channels. This application introduces a new X-ray crystal structure for the EAG1 cyclic nucleotide homology domain (CNBhD) and proposes experiments directed by structural considerations to probe its function as an allosteric modulator of channel gating. Molecules interacting with this domain and modifying channel behavior will be identified through a layered screening approach. A range of biochemical and functional approaches reflecting complementary strengths of the participating laboratories will be employed. This project is expected to address a long-standing question in the field regarding the structure and function of CNBhD's, and produce reagents that will ultimately help define the functional role of EAG1 channels in the brain and facilitate chemotherapeutic drug development.